During the past few years, we have investigated the structure of several drug-nucleic acid crystalline complexes by single crystal X-ray analysis. This work has provided valuable information concerning the molecular associations of these drugs with DNA. For example, actinomycin, ethidium and 9-aminoacridine form crystalline complexes with dinucleotides and mononucleosides, and these structures demonstrate interactive binding. Detailed molecular modes have been constructed to pinpoint the nature of these drug interactions with DNA. We are currently continuing on with this approach to attempt to understand the manner in which other drugs bind to DNA. BIBLIOGRAPHIC REFERENCES: "Concerning the Stereochemistry of Strand Equivalence in Genetic Recombination", Henry M. Solbel in Mechanisms in Recombination, edited by R.F. Grell, Plenum Publishing Company, New York (1975). "X-ray Crystallographic Visualizaton of Drug-Nucleic Acid Intercalative Binding: Structure of an Ethidium-Dinucleoside Monophosphate Crystalline Complex, Ethidium: 5-Iodouridylyl (3' -5') adenosine", Chun-che Tsai, Shri C. Jain, Henry M. Sobel, Proc. Nat. Acad. Sci. USA 72, No. 2, 628-632 (1975).